For example, Published Japanese Translation of PCT Application No. 08-500931 (JP-T-08-500931), Japanese Patent Application Publications No. 2005-32652 (2-A-2005-32652), No. 2005-19331 (JP-A-2005-19331), and No. 2005-166498 (JP-A-2005-166498) describe fuel cell systems (circulation-type fuel cell systems) in which anode-off gas, which is anode gas that has been used in the fuel cells, is recirculated to utilize any unreacted hydrogen that remains in the anode-off gas. In such circulation type fuel cell systems, nitrogen that enters the anode through the electrolyte membrane of each fuel cell is also circulated together with the anode gas, and this nitrogen accumulates in the circulation passage of the anode gas during the operation of the fuel cell system. Accordingly, a discharge valve is provided to facilitate the expulsion of nitrogen from the circulation passage. Thus, by controlling this discharge valve appropriately, nitrogen that has accumulated in the circulation passage may be expelled, so that the hydrogen concentration in the anode gas supplied to the fuel cells is recovered.
However, because nitrogen is evenly dispersed in the anode gas in the circulation passage, if the discharge valve is operated to expel a large amount of nitrogen, a large amount of hydrogen will also be expelled together with nitrogen. Even if the power generation efficiency of the fuel cell system is increased by reducing the nitrogen concentration in the anode gas, the waste of hydrogen immediately leads to a decrease in the fuel economy of the entire system. As such, there is a limit to the extent to which the nitrogen concentration in the anode gas can be reduced, and thus such improvement of the power generation efficiency is limited.
Meanwhile, there are circulation-type fuel cell systems in which power is generated at each fuel cell using anode gas retained in the fuel cell and additional anode gas is supplied to the fuel cell to replenish the anode gas used for the power generation (dead-end type fuel cell systems). Further, fuel cell systems have been under development in which power is generated at each fuel cell while expelling anode-off gas from a closed anode-gas passage at a very low rate (“continuous low-rate discharge type fuel cell systems”).
In the dead-end type fuel cell systems and the continuous low-rate discharge type fuel cell systems, too, nitrogen enters the anode through the electrolyte membrane in each fuel cell, as in the circulation type fuel cell systems. However, in the dead-end type fuel cell systems and the continuous low-rate discharge type fuel cell systems, the nitrogen that has entered the anode through the electrolyte membrane is not dispersed uniformly, instead; the nitrogen accumulates in the downstream side of the anode. Thus, in dead-end type fuel cell systems, if the discharge valve is opened after a sufficient amount of nitrogen has accumulated in the downstream portion of the anode, a large amount of nitrogen is expelled at one time without wasting hydrogen. Meanwhile, in continuous low-rate discharge type fuel cell systems, the nitrogen that accumulates on the downstream portion of the anode may be discharged little by little from the closed anode-gas passage. As such, in dead-end type fuel cell systems and continuous low-rate discharge type fuel cell systems, the concentration of nitrogen in the anode gas may be kept low, and therefore a high power generation efficiency can be achieved.
The electrolyte membrane of each fuel cell requires water molecules to enable hydrogen ions to move therein, and thus the electrolyte membrane exhibits a high hydrogen-ion-conductivity only when it is sufficiently moist. For this reason, if the electrolyte membrane is insufficiently hydrated due to a shortage of moisture in the fuel cell, the conductivity decreases, which significantly reduces the power generation performance of the fuel cell. Thus, in order to maintain a high power generation performance, it is important to keep the inside of each fuel cell sufficiently moist.
At the cathode side of each fuel cell, water is produced by hydrogen ions that have moved from the anode through the electrolyte membrane and oxygen in the cathode gas, and a portion of this water moves to the anode through the electrolyte membrane. Therefore, if this water can be dispersed throughout the anode, the entire portion of the anode may be hydrated to a moderate degree.
If there is a flow of anode gas in the anode-gas passage in each fuel cell, the anode gas flow carries water, so that it is dispersed throughout the anode. However, in the case of the dead-end type fuel cell systems and the continuous low-rate-discharge type fuel cell systems described above, the flow of anode gas in the anode-gas passage is very weak, in particular, there is substantially no gas flow at the downstream portion of the anode. Thus, in such fuel cell systems, water may not be dispersed adequately in the anode of each fuel cell, resulting in the anode being unevenly hydrated. If some portions of the anode of each fuel cell are insufficiently hydrated, anode reactions do not properly occur at said portions of the anode, deteriorating the power generation performance of the fuel cell and adversely affecting the durability of the electrolyte membrane of the fuel cell.